1. Field of the Invention
The present invention relates to a power steering device for vehicles such as automobiles or motorcars.
2. Discussion of the Related Art
Heretofore, as steering devices of vehicles and particularly, as those of automobiles or motorcars, there have been widespread power assist mechanisms (power steering devices) for steering manipulation. For this reason, these days such power steering devices are installed on almost all of the motorcars manufactured in factories. Therefore, it is often the case that the repair or replacement of such power steering devices is done at car dealers regardless of whether new cars or used cars. Generally, such power steering devices are of the type that the steering torque applied to a handle shaft through a steering handle is detected by torque detection means and that a drive power corresponding to the detected torque is generated by assist power generating means such as an electric motor and is transmitted to steerable wheels, as exemplified in Japanese unexamined, published patent applications No. 2000-168597 and No. 2001-341656.
By the way, in assembling or inspecting the aforementioned power steering devices in the manufacturing stage in factories or in repairing or replacing the same at the stage of car dealers, cares have to be taken to the following respects.    (1) Residual torque remaining as hysteresis between the steering handle and the torque detection means; this means the torque which is generated as imperfect returning of the steering handle by the cause of friction or the like in a transmission system between the steering handle and the torque detection means even after the applied torque has been removed.    (2) Origin torque caused by the difference in origin position between a pair of angle detection sensors in the case that the torque detection means is composed of a torsion bar and the pair of angle detection sensors installed apart in the axial direction of the torsion bar. In the case of a pair of angle detection sensors like, e.g., twin resolvers as disclosed in Japanese unexamined, published patent application No. 2001-194251, it is quite difficult in ordinary manufacturing sites to assemble both sensors with the mounting angle positions thereof being in completely coincidence with each other, and thus, it is unavoidable that the torque detection means detects and indicates a torque which includes the torque (origin torque) depending on the difference between origin positions of the both sensors.
Accordingly, when the steering handle is stationarily held at a neutral position in the steering direction with the applied torque having been removed in order to perform the initial setting of steering control of the power steering device, the torque detection means detects and indicates an initial torque which includes an origin torque corresponding to the assembling error in manufacturing and a residual torque generated due to the imperfect returning of the steering wheel. Of these torques, the origin torque cannot be eliminated unless the torque detection device is disassembled and adjusted, but exists as a peculiar value (i.e., constant value) for the device to be compensated easily. Therefore, the origin torque is relatively small in the influence upon the regular steering control insofar as the value thereof is specified at the time of the initial setting. With respect to the residual torque, on the contrary, where the same remains involved in the detection value of the initial toque at the time of the initial setting, the residual torque is liable to cause wrong steering control when the imperfect returning of the steering handle is corrected upon shifting to the normal steering control state. Thus, a power steering device of the rack assist type wherein the assist power generating means and the torque detection means are provided close to the steering shaft (rack shaft) as shown in the aforementioned Japanese '597 application becomes longer in distance between the steering handle and the torque detection means and therefore, is liable to be more influenced by the residual torque than that of the column assist type wherein the assist power generating means and the torque detection means are provided on a steering column, as shown in the aforementioned Japanese '656 application.
Whether the power steering devices are of the rack assist type or of the column type, heretofore, the assembling and inspection of the power steering devices in factories or the repair and the replacement of the same in car dealers have involved the following operations (i), (ii) and (iii) done by hand (by worker).    (i) The operation for holding the steering handle stationarily at the neutral position in the steering direction.    (ii) The operation for reducing (or eliminating) the residual torque existing as hysteresis, by rotating the steering handle about the handle axis a minute angle as the steering handle turns over alternatively.    (iii) The operation for detecting the initial torque by torque detection means with the steering handle being held stationarily at the neutral position in the steering direction.
However, where the aforementioned operation (ii) is performed by an unskillful worker, it is often the case that the elimination (reduction) of the residual torque becomes insufficient (imperfect) thereby to permit the residual torque to remain (included) in a detection value of the initial torque. This causes the steering control device to perform a wrong operation upon shifting to the regular steering control operation.
Further, even where the aforementioned operation (ii) is carried out reliably,. the detection of the initial torque cannot be done normally for the following reasons if the worker fails to perform the operation for holding the steering handle stationarily at the neutral position in the aforementioned operation (iii). This also causes the steering control device to perform a wrong operation upon shifting to the regular steering control operation. That is, the gravity center of the steering handle is usually eccentric (offset) from the axis of the handle axis, because it is usual that the steering handle is attached to a handle shaft to be inclined to the same and that due to the provision of an airbag or the like on the steering handle, the gravity center of the steering handle is not on the center of the handle ring. For this reason, in many cases, the gravity center of the steering handle is on the symmetrical position in the right-left direction, but is eccentric toward the driver side in the front-rear direction. Accordingly, where the initial torque is detected without holding the steering handle stationarily at the neutral position in the steering direction, the initial torque is detected as that to which the weight of the steering handle has been added as an applied torque.
Further, in order to reduce (ideally, to eliminate) the residual torque as the same becomes as small as zero in the aforementioned operation (ii), there is needed a long adjustment work by a skillful worker. This disadvantageously gives rise to problems such as an increase in the manufacturing cost, the adjustment error in car dealers due to the short of skillful mechanics and the like in installing power steering devices as standard accessories on vehicles.